1. Field of the Invention
The present invention relates to a data communication system and, in particular, to a video data communication method and apparatus which improves video quality at a recipient device and transmission efficiency by transmitting video data using both contention free transmission mechanism and priority-based transmission mechanism.
2. Description of the Related Art
Over the past few years, wireless local area networks (WLANs) based on the IEEE 802.11 specifications broadly adopted for Internet access in home and business. Early versions of IEEE 802.11 standard did not support classification of user data and priority. In order to secure Quality of Service (QoS), IEEE 802.11e proposed in 2005 introduced enhanced media access control (MAC) characterized by a hybrid coordination function (HCF). Within the HCF there are two access mechanisms, the enhanced distributed channel access (EDCA) and HCF controlled channel access (HCCA).
The EDCA provides differentiated and distributed access to the wireless medium with 4 access categories (AC) such that the data streams are transmitted in accordance with their priorities, and the HCCA guarantees reserved bandwidth for packets. Accordingly, an access point (AP) of IEEE 802.11e network can support both the asynchronous transmission and synchronous transmission.
Recently, H.264 as a new video coding standard is expected to be adopted in many applications.
The H.264 is a video compression algorithm of which root lie in the ITU-T's H.26L project and called in another name of MPEG-4 Advanced Video Coding (AVC). The H.264 promises significantly higher compression than the earlier standards. This standard achieves higher compression efficiency up to a factor of two over the MPEG 2 and enhances video quality. The H.264 also can provide DVD quality level video below 1 Mbps transmission rate so as to satisfy the requirements of transmitting a high quality data through wireless, satellite, ADSL, etc.
FIG. 1 is a diagram illustrating an H.264 compressed video data transmission mechanism over an 802.11 standard network.
Referring to FIG. 1, video data is H.264 coded, at a video coding layer (VCL), so as to be output in the form of slices, i.e., parameter set information slices, Intra (I) slices, IDR picture (I) slices, Predicted (P) slices, and Bi-predicted (B) slices. The slices are processed, at a Network Abstraction Layer (NAL), to adapt networks, then output in the format of Real Time Protocol data unit. The network-adapted video data are then transmitted in accordance with IEEE 802.11 MAC under the rule of a Distributed Coordination Function Inter frame space (DIFS) without applying per slice mapping algorithm.
In such video data transmission scheme, however, the H.264 coded video data transmission is performed without consideration of capabilities of recipient devices, whereby video quality is not guaranteed at the recipient device.
FIG. 2 is a diagram illustrating another convention H.264 compressed video data transmission scheme over an 802.11e standard network. In this video data transmission scheme, the H.264 coded video data are mapped to a single EDCA entity to be transmitted to IEEE 802.11e network. The H.264 coded data are formatted in association with RTP. Each packet slice passes the NAL and network and transport layers, and is assigned a NAL reference indication (nal_ref_indication; NRI). Next, the packet slice is mapped to an AC_VI as an EDCA entity (or Access Category; AC) and then transmitted under the control of MAC of IEEE 802.11e.
Unlike FIG. 1, in the video data transmission scheme of FIG. 2, each of I, P, and B slices is assigned a Traffic Identifier (TID) and a NRI. In FIG. 2, the slices are assigned AC_VI as the access category and 5, 4 as the TID. Also, the I, P, and B slices are assigned NRIs of 3, 2, and 1, respectively. The Access Category is contained in a header of the packet slice, and TID and NRI are contained in a frame header of the IEEE 802.11e MAC frame. Referring to the Access Category and TID, it is possible to determine how to transmit the packet data.
FIG. 3 is a diagram illustrating another conventional H.264 compressed video data transmission mechanism over an 802.11e standard network. In this video data transmission scheme, packet slices are mapped to different EDCA entities in consideration of priorities of the packets slices.
Referring to FIG. 3, the H.264 coded video data are treated only using the EDCA, and different EDCA entities (i.e. AC_VO, AC_VI, AC_BE, AC_BK) are used in accordance with priority of each packet slice.
The I slice is assigned the AC_VO (voice) which is the first priority CA and TID of 5, 4; P slice is assigned the AC_VI (video) which is the second priority CA and TID of 0, 3; and B slice is assigned the AC_BE (best effort) which is the third priority CA and TID of 1, 2. An I-frame has an NRI of 3, P-frame has an NRI of 2, and B-frame has an NRI of 1.
In the cases of video data transmission schemes of FIGS. 2 and 3 that use only EDCA, the EDCA provides class-based differentiated QoS to the IEEE 802.11 WLAN. However, EDCA is very sensitive to the increase in the number of stations, and EDCA throughput quickly degrades as the number of stations increases.
That is, when multiple stations are connected to an Access Point (AP), the EDCA can control the priorities of the traffic, but it does not support a minimum bandwidth guarantee and transport delay time. This is becoming a significant factor degrading the service quality in real time video conference services.
Since the IEEE 802.11e shares a physical layer of the IEEE 802.11a/b/g which controls the transmission rate on the basis of channel status between an AP and stations, the data rate varies in a range of 2 Mbps˜54 Mpbs according to the movements of the stations in the service area of the AP. Such data rate variation makes difficult to guarantee a fixed bandwidth for a multimedia application service. For this reason, a data rate control mechanism for guarantee a minimum bandwidth for specific service together with the location and received signal strength based on the data rate control.
Since the change of data rate makes a significant influence to a multimedia application service quality as described above, there is a need for an improved mapping technique, between the multimedia application service layer and the network layer, that can minimize the influence of the variation of the bandwidth at the physical layer.